The invention relates to shock barriers for explosives, such as shaped charges and other types of explosives used in wellbore applications.
To complete a well, one or more formation zones adjacent a wellbore are perforated to allow fluid from the formation zones to flow into the well for production to the surface or to allow injection fluids to be applied into the formation zones. A perforating gun string may be lowered into the well and one or more guns fired to create openings in casing and to extend perforations into the surrounding formation.
A perforating gun typically includes a gun carrier on which multiple shaped charges are mounted. One type of shaped charge is the capsule shaped charge, which is sealed by a capsule to protect explosive material from corrosive fluids and elevated temperatures and pressures in the wellbore. Other types of shaped charges include non-capsule charges that are carried in sealed containers or hollow carriers.
Referring to FIG. 1, a generally conical shaped charge 10 includes an outer case 12 that acts as a containment vessel designed to hold the detonation force of the detonating explosion long enough for a perforating jet to form. Common materials for the outer case 12 include steel or some other metal. With a capsule charge, the outer case 12 may be part of the capsule housing, and a cap (not shown) is attached to the front of the case 12 to keep the explosive 16 and generally conical liner 20 sealed from the wellbore environment. A non-capsule charge may be arranged as illustrated in FIG. 1, with the liner 20 exposed.
The main explosive charge 16 is contained inside the outer case 12 and is sandwiched between the inner wall of the outer case 12 and the outer surface of the liner 20. A primer column 14 is a sensitive area that provides the detonating link between a detonating cord 15 (attached to the rear of the shaped charge) and the main explosive charge 16. A detonation wave traveling through the detonating cord 15 initiates the primer column 14 when the detonation wave passes by, which in turn initiates detonation of the main explosive charge 16 to create a detonation wave that sweeps through the shaped charge 10. The liner 20 collapses under the detonation force of the main explosive charge 16. Material from the collapsed liner 20 forms a perforating jet that shoots through the front of the shaped charge 10, as indicated by the arrow 22.
The diameter and depth of a perforation tunnel created in a well formation is determined by the speed and geometry of the perforating jet as it enters the formation. The symmetry and stability of the perforating jet, which are important to promote a long straight perforation tunnel, may be adversely affected by shock waves generated by detonation of neighboring charges. As a perforating jet enters the surrounding wellbore liquid, the jet creates a cavity inside the liquid. The shock waves from the charge itself and from surrounding charges can collapse the cavity so that the liquid can interfere with the jet.
To reduce charge-to-charge interference, some predetermined separation is needed between shaped charges in a perforating gun. In conventional systems, perforator performance decreases with increasing shot density (above some critical value of shot density) and with increasing gun-to-casing clearance (the amount of water or other liquid the perforating jet has to traverse). The performance decrease is typically greater for perforating systems with capsule charges because of the direct coupling of the exploding charge case to the wellbore fluid. The cause of the performance degradation may be due to the interaction between explosive induced shock in the wellbore fluid and either the perforating jet or the perforator itself during formation of the jet.
Another issue associated with perforating and other types of explosive systems is the potential for damage to downhole equipment. For example, the perforating gun itself, the casing, and other components may be damaged by the shock induced by an explosion.
Another type of interference is xe2x80x9cpre-shockxe2x80x9d interference, in which the detonation wave traveling through a detonating cord (e.g., the detonating cord 15 in FIG. 1) interferes with the performance of the shaped charge. The strand of detonating cord 15 may be attached to a plurality of shaped charges that are mounted on the gun carrier. For a single-directional perforating gun, such as a 0xc2x0-phased perforating gun, the strand of detonating cord 15 extends generally along a straight line. The shaped charges may also be mounted in a phased arrangement, such as a spiral arrangement or some other phasing pattern. With shaped charges arranged in a spiral arrangement, the detonating cord extends in a generally helical fashion. In some other phased arrangements, such as a xc2x145xc2x0 twisted arrangement, the detonating cord 15 may be weaved in a fairly tortuous path across the rear surfaces of the charges. In all these arrangements, the detonating cord 15 traverses across substantial parts of the rear surfaces of the outer case 12 of the shaped charges 10.
As illustrated in FIG. 1, the detonating cord 15 makes contact with, or is in near proximity to, a substantial portion of the rear surface of the shaped charge 10. A detonating wave travels through the detonating cord 15 at high speed, typically about 6-8 km/s (kilometers per second). The detonation wave transfers energy to the primer column 14 to detonate the shaped charge 10. However, the detonation wave also transfers a high pressure shock, referred to as pre-shock, to the portion of the outer case 12 in contact with or in close proximity to the detonating cord. The pre-shock may also be transferred from the detonating cord to the outer case 12 through a liquid (such as water in the wellbore). Since the outer case 12 is typically made of a metal such as steel, which is a material having high shock transmissibility, the shock transferred to the explosive 16 may be significant.
Thus, an instance in time before the initiation energy of the detonating cord 15 reaches the primer column 14, a pre-shock may have been applied through the outer case 12, which is communicated into the explosive 16. The propagation of the pre-shock wave through the outer case 12 and the explosive 16 may interfere with the initiation front from the primer column 14 into the explosive 16. This may cause an asymmetry in the resultant collapse of the shaped charge liner 20. Possible adverse effects of such pre-shock interference may include one or more of the following: the perforating jet may have a crooked (rather than a straight) tip, and the cross-section of the jet may be elliptical rather than generally circular. Such adverse effects may reduce the penetration depth of a perforating jet produced by the shaped charge.
In some more severe situations, particularly with insensitive explosives having relatively slow detonation speeds, a mis-fire may occur due to the pre-shock wave reaching the explosive 16 through the outer case 12 before the main initiation front through the primer column 14. In this case the pre-shock wave densities the explosive 16 before the main initiation front reaches the explosive 16, which may cause the mis-fire.
Some conventional methods of reducing unwanted pre-shock may include the following. A separation gap may be provided between the detonating cord and the outer case. Another solution is to provide a longer primer column 14. The thickness of the outer case 12 may also be increased to increase the length of the path that the pre-shock wave has. to traverse before encountering the explosive 16 of the shaped charge. Another solution involves reducing the amount of explosive in the detonating cord to reduce the pre-shock level. Another technique is to use a detonating cord with conventional plastic jackets of standard thicknesses instead of metal jackets. Although such solutions reduce the effects of shock to some degree, they may not be adequate in some cases. For example, if the shaped charges are shot in liquid, which is usually the case in a wellbore, the pre-shock effect is accentuated since the coupling of shock between the detonating cord and the shaped charge is stronger. The shock coupling is stronger in liquid due to inertial confinement and the mass of the liquid.
A further issue associated with the use of explosives in a downhole environment is the structural integrity of the gun and attached explosives. Explosives such as shaped charges are contained or attached to gun carriers for conveying into a wellbore. The gun carriers may include strips, brackets, and the like, for carrying capsule shaped charges. Since the capsule charges are typically exposed, damage to the gun may occur when the shaped charges collide with other downhole structures as the gun is run downhole. Providing a hollow carrier may provide protection for the shaped charges and carrier of the gun, but the hollow carrier increases the outer diameter of the gun and may reduce gun performance, as measured by perforation penetration depth or the diameter of the perforation.
A need thus continues to exist for improved methods and apparatus to overcome limitations of conventional tools that contain explosives.
In general, according to another embodiment, a perforating device for use in a wellbore comprises a detonating cord, an explosive proximal the detonating cord, and a shock barrier positioned between the detonating cord and the explosive.
Other features and embodiments will become apparent from the following description, from the drawings, and from the claims.